I'm getting the "missing a using directive or assembly reference" and no clue what's going wrong

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I'm trying to allow a user to enter data into a textbox that will be added to the web.config file. I've added the relevent lines to the web.config file but when I make this class all goes wrong.

I keep getting the are you missing a using directive or assembly refenrence error whenever I try to run my app. I have looked at the other times this question has been asked and can't seem to figure out where I'm going wrong. The thing is that I am extremely new to Visual Studio and am just left blank at what could be the answer.

Below here is the class file that's generating the error. I hope I've included everything you need to assist me. Thank you.

 using System.Collections.Generic;  using System.Linq;  using System.Configuration;    namespace WebConfigDemo {     public class CompanyConfigSection : ConfigurationSection    {            [ConfigurationProperty("", IsRequired = true, IsDefaultCollection = true)]        public CompanyConfigCollection Companies            {         get                    {                       return (CompanyConfigCollection)this[""];                    }                     set                     {                         this[""] = value;                     }            }    }      public class CompanyConfigElement : ConfigurationElement    {                       [ConfigurationProperty("id", IsKey = true, IsRequired = true)]             public int Id                         {                                 get                       {                                         return (int)this["id"];                                 }                                 set                                {                                        this["id"] = value;                           }                     }              [ConfigurationProperty("name", IsRequired = true)]             public string Name             {                     get                    {                                 return this["name"].ToString();                     }                    set                    {                            this["name"] = value;                    }          }    } '  public class CompanyConfigCollection : ConfigurationElementCollection     {            protected override ConfigurationElement CreateNewElement()      {               return new CompanyConfigElement();            }            protected override object GetElementKey(ConfigurationElement element)          {                   return ((CompanyConfigElement)element).Id;             }    }     public class CompaniesConfig    {                 private static readonly Dictionary<int, CompanyConfigElement>                  Elements;              static CompaniesConfig()            {                          Elements = new Dictionary<int, CompanyConfigElement>();                        var section = (CompanyConfigSection)ConfigurationManager.GetSection          ("companies");                            foreach (CompanyConfigElement system in section.Companies)                             Elements.Add(system.Id, system);            }             public static CompanyConfigElement GetCompany(int companyId)            {                                   return Elements[companyId];        }                   public static List<CompanyConfigElement> Companies                     {                           get                 {                     return Elements.Values.ToList();                  }                   }   } }  '

Any help is appreciated